Not Applicable
Not Applicable
1. Field of the Invention
This invention pertains generally to vertical directional optical couplers, and more particularly to fused vertical couplers having a short coupling length for use in switches, filters and other electro-optic devices.
2. Description of the Background Art
Compact semiconductor optical waveguide switches are critical components in photonic integrated circuits for high speed optical communication networks and optical information processing. In many instances a large scale switching array requires low space consumption; therefore, it is essential to minimize the length of each switch. However, conventional directional couplers with laterally arranged waveguides cannot achieve the very short coupling lengths required in such applications because of low modal overlap and because of technological limits to getting uniform and small gap layers.
Vertical directional couplers, on the other hand, offer a short coupling length which can be less than 100 xcexcm. Because of their very short coupling length, as well as the feasibility of integration with other optoelectronic devices, vertical directional couplers are attractive candidates to realize photonic switches and narrowband filters. The difficulty of separating the two vertical coupled waveguides into two distinct inputs and outputs, however, has limited the application of vertical directional couplers to large scale switching arrays. In conventional vertical couplers, the two input or output waveguides are so close together that direct coupling of individual waveguides with fibers is very difficult. This drawback has limited the practical applications of vertical couplers in fiber optic systems.
Integrated compact and narrowband optical filters are key components for dense wavelength division multiplexing (DWDM) systems as add/drop multiplexers and demultiplexers. To date, many types of add/drop filters have been proposed and realized including diffraction gratings, arrayed-waveguide gratings, Mech-Zehnder interferometers and directional couplers. Compared to other structures, asymmetric directional coupler filters using two dissimilar waveguides on III-V semiconductors are promising because of the precise control of waveguide thickness and indices during crystal growth and monolithic integration with other devices such as optical amplifiers, photodetectors, modulators and lasers. There are, however, several obstacles to using these conventional vertical coupler structures in system applications. For example, the characteristics are strongly polarization dependent. Launching the light into and coupling light out of two very close waveguides is very difficult and the coupling efficiency for two dissimilar waveguide geometries can be very different.
It is well known that the response bandwidth of an asymmetrical directional coupler is inversely proportional to both the device length and the difference of mode dispersion in the two waveguides       σ    =                            ⅆ                      n            1                                    ⅆ          λ                    -                        ⅆ                      n            2                                    ⅆ          λ                      ,
where n1 and n2 are effective indices of the two waveguide eigenmodes. To minimize the device length and reduce the sidelobes, the filter bandwidth can only be narrowed by increasing "sgr". The modal dispersion depends on two factors. The first factor is the waveguide dispersion that depends on waveguide geometry; the other factor is the material dispersion. Vertical coupler filters heretofore realized primarily use waveguide dispersion difference. A narrow bandwidth requires one of the waveguides to have a very small index difference between the core and the cladding, and a large core size, while the other one should have a large index difference and a small core size. To keep the single mode operation and a high coupling efficiency with fibers, the waveguide core size cannot be too large or too small, and this limits the bandwidth of the filter. On the other hand, the difference in effective indices between TE and TM modes for a waveguide with a large index difference and small core is much more than the TE and TM difference of another waveguide with small index difference and large core size. Consequently, these devices have a strong polarization dependence. Generally the polarization dependent waveguide shift is more than 30 nm, which is a disadvantage in fiber optic communication systems. Birefringence compensation is a known technique to solve this problem, but such a solution requires a complicated structure design and a critical material growth. Since the polarization dependence and different coupling efficiency come from the strong asymmetry of two waveguide geometries, these problems can be solved if the two waveguides have similar structures.
For waveguides with almost identical waveguide dispersions, a large material dispersion difference between two waveguides is needed to realize a narrowband polarization independent filter. It is known that a material has strong dispersion when the operation wavelength is near the bandgap. Therefore, InGaAsP material can have much higher dispersion than AlGaAs material; such as approximately 1.55 xcexcm and 1.3 xcexcm. For example, the material dispersion of InGaAsP (xcex9g=1.45 xcexcm) at 1.55 xcexcm is xe2x88x920.48/xcexcm. This is nearly one order of magnitude higher than that of Al0.1Ga0.9As which is xe2x88x920.059/xcexcm. Such a large dispersion difference is very difficult to obtain if one only uses different waveguide geometries. Unfortunately, because of the large lattice mismatch, good quality InP cannot be grown on GaAs substrate or vise versa. For conventional vertical coupler filters, the difficulty of separating the two waveguides limits its application to WDM systems.
While integrated wavelength dependent couplers based on dissimilar materials are known, the devices are limited by the difference in dispersion that can be obtained with two dissimilar waveguides. Future fiber optic systems require increased levels of integration and a technology for compact WDM filters is needed.
This foregoing problems are solved with the fused vertical coupler (FVC) of the present invention which typically has a very short coupling length, such as on the order of 62 xcexcm.
By way of example, and not of limitation, a fused vertical coupler according to the present invention comprises a lower InP substrate, followed by an approximately 0.5 xcexcm InGaAsP (xcex=1.3 xcexcm) guiding layer, an approximately 0.1 xcexcm InP cladding layer, an approximately 0.4 xcexcm InP coupling layer, an approximately 0.1 xcexcm InP cladding layer, an approximately 0.5 xcexcm InGaAsP (xcex=1.3 xcexcm) guiding layer, and an upper InP substrate. The InP layers can be doped, or undoped, although doping the layers facilitates fabrication of switches and filters.
A example of a method of fabricating a fused vertical coupler according to the invention, comprises the following steps. First, a wafer is grown by MOCVD or the like. The wafer generally comprises an InGaAsP guiding layer on an InP substrate, followed by an InP cladding layer, an InGaAsP etch stop layer, and an InP coupling layer. Two samples are then cleaved from the grown wafer. In the first sample, the top InP coupling layer is removed. On the second sample, a ridge waveguide structure is fabricated using standard photolithography and selective wet etching. The facets of the two samples are aligned, mated and then fused together using high temperature fusing bonding to form the coupler.
As noted, a vertical coupler according to the present invention is fabricated using wafer fusion (also called wafer bonding). Wafer fusion is a powerful technique to fabricate structures that cannot be realized by conventional epitaxial growth and processing, and permits the joining of materials with different lattice constants. In addition to the inherent advantages of combining material with different lattice constants, wafer fusion can give an extra degree of freedom in the design and fabrication of 3-D photonic devices. For example, by displacing the input and output waveguides vertically in different planes, scaling of switch arrays to large sizes is easy to realize. For purposes of such switch arrays, a PIN straight fused vertical coupler would be used.
In order to switch large number of input waveguides, it is essential to have compact, high extinction ratio fused vertical couplers. One important advantage of the strong vertical coupling provided by fused couplers of the present invention is the capability of attaining highly selective WDM add/drop multiplexers by making longer couplers with significant overcoupling. The fused vertical couplers and waveguides of the present invention give the added advantage of vertical dimension by separating the input and output waveguides to realize compact and scalable 3D directional coupler structures.
Furthermore, because of the short coupling length of fused vertical couplers according to the present invention, a vertical coupler filter (VCF) based on wafer fusion technology is made possible. With a proper design, a narrowband and polarization independence filter with two separate inputs and outputs can be easily realized which solves all of the problems with conventional coupler filters. Wafer fusion can also be used to fabricate 3-D photonic devices. For conventional vertical coupler filters, the difficulty of separating the two waveguides limits its application to WDM systems. Using wafer fusion, however, the two close inputs and two outputs can be easily separated in different planes.
An object of the invention is to is to provide a fused vertical coupler with an extremely short coupling length.
Another object of the invention is to provide a vertical coupler which uses fusion bonding during fabrication.
Another object of the invention is to provide a fused vertical coupler for joining materials with different lattice constants.
Another object of the invention is to provide a fused vertical coupler which can be used in the fabrication of 3-D photonic devices.
Another object of the invention is to provide a fused vertical coupler which permits scaling of switch arrays.
Another object of the invention is to provide a fused vertical coupler for fabricating vertical coupler filters.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.